Security seals are used to indicate that a closed container has remained closed throughout a journey or during transport to show that its contents have not been tampered with. Once fixed, such seals can only be removed by destroying them. They usually include a unique reference number to enable it to be ensured that the seal has not been removed and replaced.
Such security seals are often colour-coded to facilitate an audit trail. For example, trolleys containing alcoholic drinks loaded onto airlines include a seal of one colour when first delivered to the aircraft to indicate that they are full and have not been tampered with. Such seals are only removed once the aircraft is airborne. Before landing the trolleys now only part full and possibly including part empty bottles are once again sealed to prevent their contents being tampered with, but, this time with a seal of different colour to show that the trolley has been used.
A popular type of seal is the so-called “padlock seal” which is shaped like a padlock and so is easily recognisable as a lock or seal. Another advantage of this type of seal is that it includes a significant surface area to carry the unique reference number and any logo. Nowadays it is often required that the unique reference number is present in the form of a machine readable barcode. To enable such barcodes to be read easily there must be sufficient contrast between the marking applied to the seal and the background colour of the seal and this is not easy to achieve when the seals themselves are colour-coded.